The present invention relates to a method for converting a video signal, generated by scanning an original copy dot by dot and line by line, into a bilevel black/white signal in facsimile reproduction.
In black/white facsimile reproduction an original copy to be copied is scanned dot by dot and line by line in a scanner by means of an opto-electronic scanning means and the brightness information of the original copy is converted into a video signal.
The original may be a printed or type-written document, a hand-written text or a graphic representation in which both the background of the original copy and the information placed on it can be white, black, grey or coloured.
The video signal generated by scanning the original is amplified, converted into a bilevel black/white signal and transmitted to a receiver via a transmission channel. The recording element of the receiver controlled by the video signal generates the required copy of the original.
When a white portion of the image of the original is being scanned the scanning means provides a high video signal amplitude, scanning a black portion of the image results in a low amplitude and a grey or coloured detail in the original in a medium amplitude of the video signal.
In order to generate a bilevel signal, the different video signal amplitudes are continuously compared with a threshold signal to arrive at a decision if a video signal amplitude is to be evalued as "white" or "black" and to be converted into the white value or the black value of the bilevel signal.
There are problems in the decision-making process, particularly if the background of an original has white and coloured areas containing, at the same time, information so that the information/background contrast is low. In this case the scanning means, due to its limited resolution, will deliver a video signal with small amplitude changes which have to be recognised with the aid of the threshold signal and evaluated correctly.
It is already known from U.S. Pat. No. 3,159,815 how to compare the video signal with a constant threshold signal in order to generate the bilevel signal.
With this so-called "constant threshold" the evaluation can only be carried out quite inadequately with a coloured original.
If, for example, an original with a coloured background and black or white information content is scanned and the decision for medium amplitudes of the video signal is basically "black" the black information on coloured background is lost; if, on the other hand, the decision is exclusively for "white", the white information on coloured background will not be considered.
It is true that an operator can select the constant threshold signal with respect to favourable results before scanning proper commences, but it is not possible to set the threshold to an optimum value not involving loss of information.
From British Patent specification No. 1,188,337 a facsimile scanner with a threshold circuit is known in which the bilevel signal is obtained by comparing the video signal with a threshold signal following it dynamically. In this arrangement the dynamic threshold signal is derived from two accompanying signals.
Although the so-called "dynamic threshold" is suitable for correct evaluation of low differences in contrast in the original copy, it produces extremely poor results if there are white/grey/black transitions running in the direction of scanning. An example for this is a white-edged original copy with coloured or grey background containing black information (letters). The known threshold circuit will interpret the coloured background as "black" so that the black information is lost.
Another example is an original in which a white area is followed by coloured information and a black background. In this case, the coloured information is reproduced as "black" so that information and background merge into each other.
It would be possible to set the known threshold circuit until it is sensitive enough to evalue the background colouration as "white" and the black information on it is recognised. In this case, however, information of lesser density as, e.g. red writing on a white area would be lost.
For this reason, individual adjustment will always only make it possible to find a compromise between high sensitivity and loss of information. For such an adjustment an experienced operator is required and it is also time-consuming since test copies must be made for evaluation.
A further disadvantage of the known threshold circuit consists in that grey or coloured areas are reproduced exclusively as "black". If a facsimile receiver is equipped, e.g. with thermal print or needle print recording devices which require a certain amount of energy for recording a black image dot in each case, the recording device can easily overheat if the original copy has a large black content.
It is an object of the invention to define a method for converting a video signal, generated by scanning an original copy dot by dot and line by line, into a bilevel black/white signal in facsimile reproduction, in which coloured original copies are converted into pure black/white copies without any significant loss of information. In these original copies both background areas and information on them can be coloured, grey, black or white.